Display devices typically display color images by means of additive color mixing of three primary colors consisting of red (R), green (G) and blue (B). In other words, in color image display, each pixel in the color display device is constituted by an R sub-pixel, a G sub-pixel and a B sub-pixel representing red, green and blue respectively. Therefore, in liquid-crystal color-display panels for example, each pixel formation portion for forming a pixel is constituted by an R sub-pixel formation portion, a G sub-pixel formation portion and a B sub-pixel formation portion which control optical transmission of red, green and blue lights respectively. The R sub-pixel formation portion, the G sub-pixel formation portion and the B sub-pixel formation portion are typically implemented by color filters.
Meanwhile, there is another color configuration proposed for displaying images in color, where each pixel consists of an R sub-pixel, a G sub-pixel, a B sub-pixel and a W sub-pixel which correspond to red (R), green (G), blue (B) and white (W), respectively. In this case, a backlight is disposed behind the liquid crystal panel to provide white light, and the W sub-pixel formation portion is either not provided with a color filter or provided with an achromatic or substantially achromatic color filter. This arrangement allows to improve brightness or to reduce power consumption in the liquid-crystal color-display device. There are still other color configurations for displaying images in color where each pixel includes sub-pixels for four or more primary colors including the three primary colors of red, green and blue plus additional primary colors other than white.
The following is a list of known examples of such color configurations as described above (hereinafter called “multi-primary-color configuration”) where each pixel includes four or more sub-pixels corresponding to four or more primary colors. (In the following list, each color combination example is followed by a corresponding sub-pixel combination which constitutes a pixel.)                a) Four primary colors consisting of red, green, blue and white: R sub-pixel, G sub-pixel, B sub-pixel and W sub-pixel        b) Five primary colors consisting of red, green, blue, cyan and yellow: R sub-pixel, G sub-pixel, B sub-pixel, C sub-pixel and Y sub-pixel        c) Six primary colors consisting of red, green, blue, cyan, magenta and yellow: R sub-pixel, G sub-pixel, B sub-pixel, C sub-pixel, M sub-pixel and Y sub-pixel        d) Seven primary colors consisting of red, green, blue, cyan, magenta, yellow and white: R sub-pixel, G sub-pixel, B sub-pixel, C sub-pixel, M sub-pixel, Y sub-pixel and W sub-pixel        
Typically, in liquid-crystal color-display devices, display data which is externally supplied is of an RGB three-primary-color format even in cases where the display devices use a liquid crystal panel of a multi-primary-color configuration. Thus, if the liquid crystal panel is, for example, of a four-primary-color configuration where each pixel includes an R sub-pixel, a G sub-pixel, a B sub-pixel and W sub-pixel, the liquid crystal display device is provided with a conversion circuit for conversion of primary-color signals R1, G1, B1 corresponding to the three primary colors of RGB (hereinafter called “three-primary-color signals”) into primary-color signals R2, G2, B2, W2 corresponding to the four primary colors of RGBW (hereinafter called “four-primary-color signals”).
It should be noted here that Patent Documents 1 through 3 listed below describe techniques related to the present invention. Specifically, Patent Document 1 describes a signal processing circuit for self-emission display devices wherein each pixel is composed of four unit pixels of RGBW. Patent Document 2 describes a RGBW liquid crystal display device wherein an output brightness data for the color white is calculated from an input data corresponding to three primary colors of RGB, as well as an arrangement for using the RGBW liquid crystal display device as an RGB liquid crystal display device. Patent Document 3 also describes a RGBW liquid crystal display device wherein an output brightness data for the color white is calculated from an input data corresponding to three primary colors of RGB and the W output brightness data is used to drive a brightness-control sub-pixel.    [Patent Document 1] JP-A 2006-163068 Gazette    [Patent Document 2] JP-A 2002-149116 Gazette    [Patent Document 3] JP-A 2001-154636 Gazette